Lumbar interbody fusion is indicated in the treatment of many degenerative lumbar spinal disorders such as spinal stenosis, spondylolisthesis, trauma, and diskogenic pain of the lumbar spine. Successful outcomes after lumbar interbody fusion are dependent, in large part, on the development of bony fusion. Several operative approaches are commonly used to access the anterior column for interbody fusion, including posterior lumbar interbody fusion (PLIF), transforaminal lumbar interbody fusion (TLIF), anterior lumbar interbody fusion (ALIF), and eXtreme lateral interbody fusion (XLIF).
The interbody cage is often supplemented by posterior internal fixation to increase early postoperative mechanical stiffness and facilitate development of bony fusion. The choice of fixation method is driven by many factors, including presence of instability, bone quality, activity level of the patient, and previous interventions. Bilateral trans-pedicle screw constructs are currently the gold standard for supplemental internal fixation of lumbar interbody fusion. Pedicle screws provide multiplanar stability and have the longest clinical history in supplementing fusion. However, transpedicular fixation is associated with risks. Cerebrospinal fluid leakage (4% of patients), transient neurapraxia (2%), permanent nerve root injury (2%), deep wound infection (4% to 5%), and hardware failure (3% to 12%) are all commonly reported complications after transpedicular fixation. Furthermore, some authors have theorized that an increase in adjacent-level intervertebral disk pressures secondary to destabilization from transpedicular fixation may be a leading contributor toward the development of adjacent segment degeneration.
In contrast to the relatively high rates of complications and adverse outcomes after transpedicular fixation, spinous process fixation through plating increases stability at a reduced risk. Furthermore, spinous process plates (SPPs) may be placed with patients in the same lateral decubitus position used for some interbody procedures, avoiding the need for repositioning such as would be required with supplemental transpedicular fixation. Additionally, spinous process plating potentially shortens operative times, decreases blood loss, and reduces fluoroscopic exposure to the operating room team compared with pedicle screw fixation.
The main indication for spinous process plating is to provide supplemental fixation following any type of interbody fusion ( Fig. 19.1 ).
There are a number of limitations with the use of spinous process plating systems. Foremost is the necessary presence of the spinous processes. This would exclude their use in most revision spine procedures and in many patients who have a traumatic fracture. Owing to the relatively small size of the S1 spinous process, it is often not possible to place these implants at L5-S1. Some implants are designed specifically to accommodate the S1 process, such as the Aspen device (Zimmer Biomet, Denver, CO) featured in the surgical technique section. Spinous process plates’ use may be limited in those patients with advanced osteoporosis. In these patients, the plate may fracture or cut through the spinous process during or immediately after implantation. Consideration must also be given to the amount of fixation provided by these implants. In cases where robust fixation is needed (e.g., instability, tumor), these implants may not provide adequate stability. Lastly, it may be difficult and/or biomechanically insufficient to place these devices over multiple levels. The authors have used SPP over two levels, but never more.
Patients are positioned prone in customary fashion on a spine table. Fluoroscopy is typically used during insertion, but not required ( Fig. 19.2 ). The authors prefer a Jackson radiolucent table. Alternatively, patients may be placed in a lateral decubitus position. In this manner the surgeon may sit down and perform surgery looking straight ahead.
Following preparing and draping of the lumbar region, an incision is planned. The incision is typically slightly caudal to a typical incision planned for a laminectomy; specifically, it only is required to expose the interspinous space, which is typically slightly caudal to the disk space on lateral imaging. After an approximately 2.5- to 4-cm incision is made, the subcutaneous fat and lumbodorsal fascia are then divided and retracted. After dividing the fascia, electrocautery is used to perform a subperiosteal dissection so that the bilateral erector spinae muscles can be mobilized off of the cranial and caudal spinous processes to the spinolaminar junction. If desired, for greater visualization or wider decompression, the interspinous ligament at the target level may be removed with rongeurs. The supraspinous ligament can be left in place to facilitate a more anatomic closure, it may be resected completely, or it may be elevated and sutured back in place following placement of the implant. Portions of the spinous processes and/or laminae may also be harvested and placed in the interspinous and/or laminar space for posterior arthrodesis prior to implanting the plate. For decompression, bilateral hemilaminotomies and medial facetectomies may be performed ( Fig. 19.3 ). Care must be taken not to remove too much of the lamina in the midline or facet joint. It is difficult to quantify, but the authors recommend leaving greater than 50% of the lamina and no greater than 50% of the facet joint. This may result in fracture of the spinous process at the laminar junction at the time of implant placement or instability.
The surgeon first exposes the lamina and spinous process of the surgical levels in a subperiosteal fashion. The supra- and intraspinous processes are left in place.
Following level confirmation by fluoroscopic guidance, spinal decompression is accomplished via bilateral hemilaminotomies performed under the microscope.
The interspinous space is then dilated followed by a reaming using appropriately sized interspinous reamers.
Following sizing and reaming, the Aspen device is placed and tightened across the spinous processes.
The implant placement is confirmed by visual and fluoroscopic imaging.
The wound is closed in layers.
(Courtesy of Zimmer Biomet.)
After adequate decompression or dissection, the SPPs are inserted. The exact placement depends largely on the type and brand of implant used. For this chapter, the technique for placing the Aspen system (Zimmer Biomet) is illustrated ( ). The interspinous region is first prepared. This preparation depends on the decompression that was performed above. If the interspinous ligament was left in place, a dilator may first be placed into the interspinous space to create a pathway for the implant ( Fig. 19.4 ). Of note, hypertrophic facet joints may prevent adequate placement and overgrown bone may first need to be removed with a rongeur or high-speed drill at this point. It is imperative that the dilator is placed as anterior as possible during this step. This may be ensured with the use of fluoroscopy. Level confirmation may be made during this step as well.
Video 19.1 Aspen surgical technique. In this video, the surgeon performs a minimal access lumbar decompression and placement of the Aspen (Zimmer Biomet) spinous process plating system.
A spreader is then placed in order to measure the interspinous space. The spreader is opened, distracting the spinous processes with care not to over distract and fracture the bone ( Fig. 19.5 ). The spreader’s ratchet is kept up at this point and when proper distraction is achieved it is dropped down and a measurement may be made. There is risk of fracture of the spinous process. This is especially true in those with osteoporosis. If fractured, the spinous process plate will need to be abandoned. The authors always recommend having a back-up plan for fixation when performing this procedure.
If the supraspinous ligament was left in place, a rasp is next placed in the interspinous space ( Fig. 19.6A ). The rasp is typically one size smaller than the measured distraction on the spreader. The rasp may be rotated cranial and caudal to prepare the interspinous space for fusion ( Fig. 19.6B ). A larger rasp may then be used if desired. After completion, the corresponding sleeve is placed over the rasp and reinserted in the interspinous space ( Fig. 19.7A ). After the rasp is removed, the sleeve will serve as a cannula for the plate insertion ( Fig. 19.7B ).
The appropriate size post plate is next chosen. The diameter is based on prior sizing with the spreader and rasp. The length is based on local anatomy. The standard barrel length is typically appropriate; however, if there is significant facet hypertrophy limiting anterior placement, a medium barrel length may be chosen ( Fig. 19.8 ). The chosen plate must engage an appropriate amount of spinous process without extending beyond the bone cranially and caudally; it must be anterior. The Aspen system offers a flared design, which may allow fixation at S1, but also offers anterior placement at all levels. The post plate is placed through the cannula and the cannula then removed ( Fig. 19.9 ). The locking plate is then slid in place over the barrel engaging the opposite side of the spinous process ( Fig. 19.10 ). Once the plate is in place, compression is placed on the cranial and caudal portions of the device, driving the spikes of the plates in to the spinous processes ( Fig. 19.11 ). The spikes should be fully driven in with care not to apply too great a force, which may fracture the bone. Once adequate compression is achieved, the locking screw is finally tightened ( Fig. 19.12 ). The authors utilize fluoroscopy at this point to ensure appropriate placement ( Fig. 19.13 ). Of note, if the supraspinous and intraspinous ligaments were resected during decompression, the above procedure is performed without the need for rasping and the placement of the interspinous cannula. Following sizing, the appropriate implant may be directly placed in the interspinous space.